The equation assumes that the time it takes for messages to go from the master to the slave, the forward delay, is the same as the time it takes dub messages to go from the slave to the master, the reverse delay. Diagram of hardware timestamping during the transport of a Sync messages.
Its mainly due to all of those pesky queues. Another type, called a boundary clock uses the PTP messages to set its own clock, then sends its time to PTP slaves which need it.
Figure 1. When messages depart from or arrive at a network port, special hardware generates a timestamp from the local clock, usually in the media independent interface between the data link layer MAC and the physical layer PHY.
So obviously if this happens in the one direction, but not the other, then BAM! How this works is shown in the diagram below. As it turns out PTP has an alternative delay measurement mechanism known as the Peer-to-Peer mechanism.
Any difference in the forward and reverse delay in an error in determining the difference between the master clock and the slave clock. This delay measurement mechanism is know as the End-to-End delay measurement mechanism. Two words: Hardware timestamping.
Figure 2. At the end of these exchanges the slave clock has all four timestamps.
Let me explain. There are queues in the routers, there are queues in the switches, there are even queues in the network stacks at the end devices. Since I already gave away the ending in the opening sentence, you know that this is solved with hardware timestamping.
One type of such devices, shown below, called a transparent clock works by updating PTP messages to correct for time spent in the device. That removes the unpredictably slow response of the operating system and other software.
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In some cases the delay can be quite long, many microseconds, or even milliseconds. Why would the forward and reverse delays be different?
There is no problem if these delays are large, just so long as they are the same.